But Ofcom stepped in after it received thousands of complaints from disgruntled consumers who had been switched to another operator without prior knowledge or consent, a mis-selling tactic known as "slamming".

LLU technology uses telephones lines running between homes or businesses and the local exchange that are not connected by BT, which has allowed competition into the voice and broadband market from other providers.

Fixed-line voice call service providers have been required to comply with Ofcom's code of practice under a General Condition of Entitlement (GC 14.5) since May 2005.

The rules were originally intended to cover a two-year period, but after public consultation Ofcom said the rules will continue to apply to fixed-line, as well as being extended to cover LLU providers.

It also said it has investigated 11 providers including Tesco, Unicom, and FreeCall since 2005 and that it is currently reviewing switching methods across all telecoms services.

Ofcom chief executive Ed Richards said: "We want competition and choice to continue to grow, and we want consumers to benefit from these changes.

"Consumers need to be able to shop around with confidence. Extending these rules and our enforcement activity will protect consumers from inappropriate sales and marketing techniques."